In semiconductor packaging and assembly, it is sometimes necessary to form an electrical connection between different electronic components using conductive wire, or between different electrical contacts of an electronic component. Wire bonding is a method commonly used in the industry to form the electrical connection. One example is where a semiconductor die or integrated circuit chip is attached onto a carrier such as a leadframe. In this case, electrical connections have to be formed between electrical contacts on the die and corresponding electrical contacts on the leadframe. Thus, the wire needs to be bonded at one end to a lead of the leadframe and at the other end to a connection pad of the die. The most widely used wire materials are Gold (Au) and Aluminum (Al), but Silver (Ag) and Copper (Cu) are also used. The connection pads may comprise metallized bond sites on a semiconductor chip or on interconnection substrates or carriers.
A typical method used to bond or weld the wire to a connection pad is through a combination of heat, pressure and/or ultrasonic energy. It is a solid phase welding process, wherein the two metallic materials (the wire and the pad surface) are brought into intimate contact. Once the surfaces are in intimate contact, electron sharing or interdiffusion of atoms takes place, resulting in the formation of a wire bond. The bonding force can lead to material deformation, breaking up of a contamination layer and smoothing out of surface asperity, which can be enhanced by the application of ultrasonic energy. Heat can accelerate inter-atomic diffusion, thus forming the bond.
One type of prior art wire bond formation uses a ball bond. The process involves melting a sphere of wire material on a length of wire held by a capillary, which is lowered and welded to a first bonding position. This results in a bond with a circular smashed ball shape. The capillary then draws out a loop and then connects the wire to a second bond position using a stitch bond that is usually of a crescent shape. Another ball is then reformed for a subsequent first ball bond. Currently, gold ball bonding is the most widely used bonding technique. Its advantage is that once the ball bond is made on the connection pad of a device, the wire may be moved in any direction without stress on the wire, which greatly facilitates automatic wire bonding.
Current wire bonding techniques depend very much on the area of contact between the formed ball and the connection pad of the electronic device for adequately securing the connection. Over the years, the demand for fine-pitch bonding (such as with bond pads having pitches of less than 50 μm) has increased steadily, thus making effective bonding more difficult since there is a smaller surface area for contact between the wire bond and the connection pad. Furthermore, probe testing of semiconductor devices has become the norm. Probe testing may cause the surfaces of the connection pads to be damaged, leaving probe marks on the connection pads which might be rough or have an under-layer material exposed, thus adding to the difficulty to form an effective bond since good intermetallization is harder to achieve.
Another problem associated with fine-pitch bonding is that if an insufficient amount of ultrasonic energy or bond force is applied during bonding, ball lift occurs when the adhering force between the ball bond and the connection pad is not strong enough. Conversely, if too much ultrasonic energy or bond force is applied, this may lead to metal peel or cratering on the surface of the connection pad. Moreover, in fine-pitch ball bonding, a parameter window for forming a good bond is comparatively smaller. Therefore, the aforementioned faults would have a tendency to occur either due to the sensitivity of the connection pad of the wafer or other semiconductor device, or due to the parameters not being properly optimized.
In order to improve the intermetallization between the ball bond and the connection pad, one method is to increase the ball size. Unfortunately, the size of the ball is restricted to the size of the opening offered by the connection pad, which is smaller for smaller devices. Another method is to increase the ultrasonic energy transmitted to the ball bond during bonding. However, this method increases the risk of metal peel or cratering if the wafer or semiconductor device is sensitive.
If the adhesion of the wire to the bonding location is increased by increasing the contact surface area between the wire bond and the bonding location, this would also give rise to increased shear strength. In other words, it would require a greater force to dislodge the bond from the bonding location, so that the bond is more effective and reliable.
Using the aforesaid prior art bonding process to apply a single ball bond to the bonding pad, and relying on the adhesion of the single ball bond to the bonding pad to secure the bond, may not achieve sufficient shear strength to ensure reliability of the wire bond. It would be desirable to develop a wire bonding method and a wire bond that has increased shear strength that would meet the needs of fine-pitch wire bonding in modern wire bonding machines.